Grafts of primary fibroblasts transduced to express human nerve growth factor have been shown to sustain NGF in vivo gene expression for at least eighteen months in the rodent central nervous system. In addition, these grafts sustain NGF messenger RNA production for at least 14 months in vivo. In primate systems, ex vivo NGF gene therapy has been demonstrated to sustain NGF protein production in the brain in the rhesus money for at least one year.